A Southwest Airlines plane from Dallas landed in Houston on Friday after a written threat was found on board but officials said it did not appear to be linked to terrorism.

The Boeing 737 with 136 passengers and five crew was diverted to an isolated section of Houston Hobby Airport. All passengers were safely evacuated, an airport spokesman said.

"It doesn't appear to be terrorism at first glance," said Special Agent Al Tribble, Houston FBI spokesman.

Bomb-sniffing dogs and law enforcement officials were examining the plane and officials were questioning and screening passengers, Homeland Security officials said.

Flight 21 was going from Dallas to Houston and then on to Corpus Christi, Texas.

At about 1 p.m. (1700 GMT) a passenger on the flight found a crumpled note saying "something to the effect there's a bomb on board," Tribble said. The passenger gave the note to a flight attendant.

The crumpled nature of the note and how it was obtained makes the possibility of terrorism seem less likely, Tribble said.

"It was crumpled up," he said. "Someone at the scene said this thing could have been on the plane for two weeks."